Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Most advanced composites are fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partially cured resin. Resin systems containing an epoxide resin and aromatic amine hardener are often used in prepreg since they possess the balance of properties required for this composite fabrication process. State-of-the-art epoxy/carbon fiber composites have high compressive strengths, good fatigue characteristics, and low shrinkage during cure. However, most epoxy formulations absorb moisture which reduces their high temperature properties. As a result they are not suitable for use at 350.degree. F. or greater in a moisture saturated condition. There is therefore a need for resin systems which afford composites which can retain a high level of properties at 350.degree. F. under such moisture saturated conditions.
Most prepreg resins designed for use at 350.degree. F. are made by combining bismaleimides of Formula I with liquid co-reactants containing other reactive groups such as amines, epoxides, cyanates or co-monomers containing --CH.dbd.CH.sub.2, &gt;C.dbd.CH.sub.2, or --CH.dbd.CH-- groups which can react or polymerize with the carbon-carbon double bonds of the maleimide groups. ##STR1##
In common bismaleimides, R is the residue of an aromatic diamine such as methylene dianiline or m-phenylene diamine.
A major disadvantage of most resin formulations containing bismaleimides, however, is their generally brittle nature and, in turn, the lack of toughness and damage resistance of reinforced composites made from those resins. Bismaleimides having improved toughness, particularly as evidenced by improvements in composite properties, while maintaining high temperature performance, would provide a significant improvement over most of the currently available BMI resins.